The invention relates to a heavy-duty submersible pneumatic drilling unit.
With submersible drilling units as well as with all types of pneumatic impact equipments, the intensity of installed capacity is determined by the product of the piston impact energy and the piston motion frequency. Such parameters are given above all by the pressure intensity of supplied air, the size of active piston surfaces alternately engaged by compressed air both in the upper and the lower space of the working cylinder, by the weight and stroke of the striking piston, the applied system of filling and exhausting the working cylinder spaces and, finally, the detailed shape design of the individual equipment parts. With the given intensity of supplied air pressure, the size of active piston surfaces cannot be enhanced by enlarging the working cylinder diameter as it is usual with other types of pneumatic impact equipments. The limitation is given here by the drill hole diameter and the external diameter of the drilling unit, since between the hole wall and the unit an annular space has to be left for raising drillings by exhaust air.
Under these circumstances, practically only one known and real possibility how to markedly enlarge the active surfaces of the striking piston lies in the so-called tandem arrangement of the piston, consisting in that the working cylinder spaces adjacent the two axially arranged piston heads are doubled. With the installed capacity in view, such an arrangement is effective but technologically rather complicated and expensive. Owing to a plurality of cross-sectional changes along the tandem piston axis, a tension concentration occurs in some piston portions, if it is exposed to an impact stress. Proportionally to the piston impact speed also the stress in its critical portions rises up to such a value that at a particular impact speed the stress may exceed the fatigue strength of piston material so that a fatigue fracture may occur. This is why in case of relatively high supply air pressures and consequently high impact speeds, the tandem piston arrangement cannot be used.
Other limits in raising the installed capacity of submersible equipment include the use of a particular system of compressed air distribution. This is the system of filling and exhaust ducts for feeding compressed air into and withdrawing it out of working spaces of the cylinder, respectively. In practice, many distributing systems are used, such as plate, ring, slide and flap valve distributors. Apart from these, some systems without any separate distributing means are known, wherein the working spaces are supplied with compressed air long the surface of or through a bore in the piston. Filling, exhaust and bypass ducts are provided in the wall of the cylinder, or in its liner, in the piston or in a pin passing therethrough, or by combining the above modes. The filling and exhausting function is partially assumed by drilling bits or parts that are connected or associated with them, said parts being specifically shaped for this purpose. Needless to say, all of the embodiments as hereinabove referred to have their advantages and drawbacks which manifest themselves in technical parameters, technology, structure, price, lifetime, etc.
It is an object of all of them to optimalize the piston stroke cycle, which means to obtain the backward stroke within a desired range, to stop the piston in the upper dead center without shock, and to give it for the next impact stroke the necessary impact speed, all of this within an as short a time interval as possible and at a minimum air demand. During its backward motion, the piston makes no work, and the energy it has been given at the start gets wasted in the final phase by counterpressure. Thus in an endeavour to raise the unit output it is advisable to shorten the time interval of the backward stroke motion as much as possible and, consequently, to enhance the piston frequency. This is attainable by intensively braking the piston in its upper dead center as e.g. by compression. High air compression values prevailing in the dead center region after the filling ducts have been closed by the piston head, will not only shorten the piston braking period but give the piston at the same time a high acceleration while starting the impact stroke motion. The compression space created in the upper dead center makes it thus possible to impart to the piston during the backward stroke a higher kinetic energy, to accumulate it and effectively apply it at the start of the impact stroke. In this way it is theoretically possible to raise together with the impact frequency of the piston also the energy thereof whereby the installed unit capacity increases. In practice, however, a considerable portion of the backward stroke energy accumulated in the compression space is dissipated due to leakage between the piston and the cylinder, and to a heat removal. As the piston starts its impact stroke the compressed air expands, and at the instant of opening the compression space the pressure does not recover, owing to such losses, its original value at the start of compression drops to a substantially lower one. After the compression space has been opened during the impact stroke, the piston, due to a high acceleration, has already a considerable speed so that a relatively rapid change in the volume of the upper working space in the cylinder occurs. Under these circumstances, compressed air supplied through blocked profiles of filling ducts does not suffice to refill the upper working space of the cylinder so that during the remaining impact stroke phase this space is imperfectly supplied with compressed air. This impairs the piston velocity increase during the remaining stroke phase and negatively influences the impact speed and energy. The resulting effect of energy accumulation during the backward stroke gets lost and the efficiency of energy transfer from the backward stroke to the impact stroke drops.